A control moment gyroscope (CMG) maintains and adjusts the attitude of a satellite. A CMG usually consists of a spinning rotor and multiple motorized gimbals that tilt the rotor's angular momentum. When the rotor is displaced about a gimbal axis, the angular momentum changes and causes a gyroscopic torque that rotates the satellite.
Three or more CMGs are necessary for linear control of satellite attitude in three degrees of freedom. Two CMGs can be utilized as a scissor pair to provide control in a plane and, when utilized with another independent actuator(s), can steer a satellite in three axes. Regardless of the number of CMGs, gimbal motion may lead to relative orientations called singularities that produce no usable output torque along certain directions. When a satellite experiences a singularity, the satellite may lose control and stray from the objective orientation path.
A first class of singularity escape and avoidance methods tends to utilize an external actuator that is not part of the CMG array. The additional actuator augments the CMG array by adding more degrees of freedom and therefore singularities are avoided, or can be escaped. These methods tend to perform slowly and create other problems such as mission planning in the presence of an uncertain momentum envelope. A second class of singularity escape and avoidance methods uses mathematical augmentation or manipulation of the CMG array control law. These methods have drawbacks including having computationally intensive algorithms, not being deterministic, and creating torque disturbance.
Therefore, there exists a need for improved methods and systems for singularity escape and avoidance. 